Some exhaust sensors exposed to exhaust gas require air for avoiding contamination with exhaust gas components, or for using the air as a reference in concentration measurement of the exhaust gas components. Such air is supplied in different ways depending on characteristics of the exhaust sensors.
When the exhaust sensor is used as an on-vehicle PM sensor, the PM sensor requires air for measurement of exhaust gas components, and is typically designed on the assumption that the air is supplied to the PM sensor by positive pressure generated by an air compressor or turbo compressor.
One prior art document is WO2009/109688.